sleeper_inc

In this "how to" I'm going to show you the proper way of testing your cars compression. There are many different ways of doing this, but I'm going to show to the way that works best for me today.

First your going to want to buy a GOOD compression tester (bought mine for $20 at princess auto a few years ago)


Take the spark plug out... ONLY ONE and from only ONE SIDE if you have the 91-93 DIS engine (exhaust side since it's easy)


Next, screw the end of the compression tester into the spark plug hole. You don't have to tighten it down really hard, finger tight works just fine.


Next you'll want find a place were the pressure release valve wont be pushed down and that the tester wont be caught in something rotating.


Make sure to have the coil unplugged, because you DON'T want the car running.

Now go into the car, hold the pedal at WOT, and crank the car over for 5-10 seconds while at WOT. Once the time has passed, stop cranking the car over and go record your compression results.


Woooo 140psi.... Thats ok. My rule of thumb, if its under 110psi, it's either time for a rebuild, or your valve timing is off.

Now put the spark plug back in and move to the next one.

Heres the rest of what my test came out as.


cylinder #2


cylinder #3


cylinder #4

If you want to check your rings do the test, then drop some oil down the cylinder, and if the compression raises a fair amount, then it's safe to say your rings are going or need to be replaced.

Hope this helps you guys and gals.

samthebullitt

how does the psi convert to a comp ratio? for example, o have a bone stock 85 2.3 mustang n/a and i just had .162 shaved off the head and a 5 angle valve job done.. it pushes 175psi at on the lowest cyl.. i asked around a bit but i havent a clue.

sleeper_inc

stock is 9.5:1, and then you need to calculate the new CC's over the old CC's of the combustion chamber. It should be on google if you search (guessing 9.7:1 ratio or a little less for you)

plazma

You might also note that if you are cheching compression because you think you might have killed a head gasket then take all the spark plugs out.

Gaskets are known to blow from cylinder to cylinder and if you don't have the plugs out it can give you a false high reading. FWIW

Boost Gobbler

droping oil in the chamber is to attempt to simulate the piston seal of a piston at operating tempature. if it compression rise's to this effect it is normal.

86GT 93LX

ive always been told pull all plugs for this reason plus the motor can crank more freely.also some people say to crank it over till the gauge stops climbing in pressure then wait 10 mins and see if it drops at all so its sorta a leak down test. what is the comp supposed to be at brand new? the comp not the ratio...like what 180?

sleeper_inc

160-180psi brand new